Embodiments of the present invention provide a process for production of high-pressure gaseous oxygen and, more specifically, provide a multiple stage process that permits more energy efficient production of high-pressure gaseous oxygen.
As used herein, the term xe2x80x9cHPxe2x80x9d means and refers to high pressure. As used herein, the term xe2x80x9cMPxe2x80x9d means and refers to medium pressure and is generally used to refer to a pressure that is acceptable for a fin heat exchanger, such as a brazed aluminum plate fin heat exchanger. As used herein, the term xe2x80x9cnet powerxe2x80x9d is the power consumed by the process, such as, in an embodiment, the power consumed by the air compressors plus the power consumed by each pump. However, xe2x80x9cnet powerxe2x80x9d may be defined otherwise. As used herein, the term xe2x80x9cspecific powerxe2x80x9d is the ratio of the net power divided by the gaseous oxygen production flow and will be described in terms of Kw/Nm3, unless otherwise specified. As used herein, units for pressure will be xe2x80x9cBara,xe2x80x9d unless otherwise specified; units for temperature will be xe2x80x9cxc2x0 C.,xe2x80x9d unless otherwise specified; units for flow will be xe2x80x9cNm3/h,xe2x80x9d unless otherwise specified; and, units for power will be xe2x80x9cKw,xe2x80x9d unless otherwise specified.
It is common to produce high-pressure oxygen gas at the outlet of the cold box by internal compression. Commonly, in air separation units, liquid oxygen is extracted from a distillation column, compressed by a pump and vaporized under pressure to produce high-pressure gaseous oxygen. In order to vaporize the oxygen efficiently, it is necessary in the prior art to condense another stream, which is generally a portion of the incoming air compressed to a pressure sufficient to allow its condensation at a temperature above the vaporizing oxygen. In some cases, the pressure of the oxygen product is such that the corresponding air pressure exceeds the limits of what can be reasonably achieved with the present available technology of efficient heat exchanger technology, such as brazed aluminum plate fin exchanger.
One prior art solution has been to use a spiral wound tubular exchanger, which is able to withstand much higher pressures. However, these exchangers, contrary to plate fin exchangers, cannot accommodate multi-stream exchange in countercurrent directions, i.e. two directions. These exchangers are limited to a few streams in one direction and one stream in the other direction. In this arrangement, such as mentioned in examples found in U.S. Pat. Nos. 5,337,571; 4,345,925, processes must be adapted so that the heat exchange on the oxygen stream takes place in the exchanger in countercurrent passage with a single stream under higher pressure. The stream is typically either air or nitrogen, however, other gases are used. The resulting exchange induces a significant inefficiency, as the temperature difference between the two streams along the exchanger cannot be kept at low values.
More specifically, U.S. Pat. No. 5,337,571, discloses a nitrogen-cycle installation wherein the cycle compressor provides a supply of high-pressure nitrogen which serves to heat oxygen supplied in liquid form from the reservoir of a low-pressure column and raised in pressure by a pump to the desired high production pressure. Oxygen gas may be produced at a pressure exceeding about 50 bars.
U.S. Pat. No. 4,345,925 discloses producing oxygen gas at greater than atmospheric pressure by separating air into oxygen-rich and nitrogen-rich fractions in a distillation column, removing the oxygen as liquid and pumping it to the desired pressure and subsequently vaporizing the pumped liquid oxygen by means of energy absorbed from a recirculation argon containing fluid.
Another prior art example is found in U.S. Pat. No. 5,758,515. This patent discloses a cryogenic air separation system wherein feed air is compressed in a multistage primary air compressor, a first part is turboexpanded and fed into a cryogenic air separation plant, and a second part is turboexpanded and at least a portion of the turboexpanded second part is recycled to the primary air compressor at an interstage position.
Another prior art example is found in U.S. Pat. No. 5,655,388. This patent discloses a cryogenic rectification system wherein liquid oxygen from a cryogenic air separation plant is pressurized and then vaporized in a high pressure liquefier producing product high pressure oxygen gas and generating liquid nitrogen for enhanced liquid product production.
Another prior art example is found in U.S. Pat. No. 5,628,207. This patent discloses a cryogenic rectification system for producing lower purity gaseous oxygen and high purity oxygen employing a double column and an auxiliary column which upgrades lower pressure column bottom liquid or processes higher pressure column kettle liquid.
U.S. Pat. No. 5,901,579, the disclosure of which is incorporated herein by reference speaks to the inefficiencies of the present processes when it states xe2x80x9cFor an internal compression cycle, efficient, cost effective turndown of the liquid production from the design point cannot be achieved with conventional cycles and/or turbomachinery,xe2x80x9d in its background section The prior art solution provided by the ""579 patent was to construct a cryogenic air separation system wherein base load pressure energy is supplied to the feed air by a base load compressor and custom load pressure energy is supplied to the feed air by a bridge machine having one or more turbine booster compressors and one or more product boiler booster compressors, all of the compressors of the bridge machine driven by power supplied through a single gear case.